RDF-Entailment: Remove duplicate anonymous resources - looking for an algorithm

Hello

I'm looking for an algorithm to remove duplicate anonymous resources 
from an RDF-Model.

Fo example, given the graph:

(1) _:a rdfs:label "test".
(2) _:a rdfs:comment "...".
(3) _:b rdfs:label "test".

The algorithm should tell me that I can safely drop statement 3.

Or in a more complex example:

(1) _:a foaf:knows _:b.
(2) _:b foaf:knows _:a.
(3) _:c foaf:knows _:d.
(4) _:d foaf:knows _:c.

Tell me that 3 and 4 can be removed, but that the reduced graph cannot 
further be reduced, even if _:a and _:b seem indistinguishable.

Does anybody has an algorithm or even some code for doing this?

Thanks,
reto